The math ability mindset refers to implicit beliefs about the malleability of math ability. Some believe that math ability can be improved by putting in effort, while others believe that math ability is fixed and cannot be changed as a result of effort. Such a mindset can influence an individual’s affect, cognition, and behavior. The purpose of this study was to examine how mothers’ mindsets about math ability influence negative affect and attitude toward math (math anxiety) and math interaction with their children. For this purpose, the study focused on mothers of 5-year-old children. The research results are as follows. First, mothers with a fixed mindset tend to have a high levels of math anxiety. In other words, mothers who believe that math ability is fixed so that it cannot be changed by individual effort are likely to have a high fear of math and avoid situations involving math. Second, mothers’ mindsets predict how often they engage in daily interactions with their children while doing math. Compared to those with a growth mindset, mothers with a fixed mindset tend to engage in less daily math conversations and activities with their children. Third, mothers’ math anxiety mediates the link between mothers’ mindsets and math interactions with children. Mothers with a fixed mindset tend to have a high level of math anxiety, which in turn lowers the mother-child interaction about math. This study shows that mother’ cognitive and emotional changes towards mathematics should be adjusted in order to promote behavioral changes in mother-child interaction.